Color electrophotographic printers provide full color images by building up and sequentially transferring individual color separation toner images in registration onto a receiver and fusing the toner and receiver. Specific color outcomes are achieved in such printers by providing toner images of specific colors that, when assembled in registration with toner images having other specific colors form precise combinations of differently colored toners that have the appearance of a desired color at specific locations on a receiver. Similarly, the gloss of such electophotographcially produced color toner images can be enhanced by combining a toner image formed using a toner that will be generally transparent after fusing in registration with the color toner image to provide a layer of toner having a consistent index of refraction and optionally reduced surface roughness.
It will be appreciated that many desirable printing outcomes can be achieved through controlled combinations of different toner types. However, a central limitation on the use of multiple different toner types in electrophotographic printers and methods is that electrophotographic printing modules of the type that form the individual toner images can be large, complicated and expensive. Further, it is difficult to ensure registration of the printing modules with the transfer systems and receivers in a digital printer and such difficulties increase with each additional printing module that is to be incorporated into a printer.
Accordingly, printers are typically designed to provide a limited number of such electrophotographic printing modules. For example, the Nexpress 2100 and subsequent models provide a tandem arrangement of five printing modules. During printing of a color image four of these tandem printing modules apply different ones of four toners, each supplying one of the four primary subtractive colors, while a fifth printing module is used to apply custom colors, clear overcoats and other different types of toner to the formed toner image. While this can be done in a highly effective and commercially viable manner, there remains a need in the art for methods that enable toner images to be formed for use in making an electrophotographic print that include a greater number of different toners than the limited number that are currently available and that can provide such toners in controlled registration and in an image modulated manner.
In one alternative, U.S. Pat. No. 5,926,679, issued to May, et al., discloses that a clear (non-marking) toner layer can be laid down on a photoconductive member (e.g., imaging cylinder) prior to forming a marking particle toner image thereon, and that a clear toner layer can be laid down as a last layer on top of a marking particle toner image prior to transfer of the image to an intermediate transfer member (e.g., blanket cylinder). It is also disclosed that a clear toner layer can be laid down on a blanket cylinder prior to transferring a marking particle toner image from a photoconductive member. In one aspect of this patent, a non-imagewise clear toner layer is bias-developed on to an intermediate transfer member using a uniform charger and a non-marking toner development station. A first monocolor toner image corresponding to one of the marking toners is transferred to the ITM (on top of the clear toner) from a primary imaging member which may be a roller or a web but is preferably a roller. Subsequently, a second monocolor toner image corresponding to another of the marking toners is transferred to the ITM (on top of and in registration with the first toner image) and so forth until a completed multicolor image stack has been transferred on top of the clear toner on the ITM. The ITM is then positioned at a sintering exposure station; where a sintering radiation is turned on to sinter the toner image for a predetermined length of time.
However, while this approach can be effective and can provide a commercially viable solution, this approach requires an additional transfer step for each toner that is applied which, in turn, reduces machine productivity.
Accordingly, what is needed in the art are printers and printing methods that enable an increase in the number of toner types that can be provided to form a color toner image without compromising the efficiency and the accuracy of registration with which each of the toners can be provided.